RFR: 8310049: Refactor Charset tests to use JUnit [v3]

[Justin Lu]

> As discussed in https://github.com/openjdk/jdk/pull/14473/files, tests within _test/jdk/java/nio/charset/Charset_ could benefit from using a test framework such as JUnit. 
> 
> In addition, this PR groups the emptyCharset, nullCharset, and defaultCharset tests into _illegalCharsets.java_. The _default.java_ file was removed, as it did not test anything.

Justin Lu has updated the pull request incrementally with eight additional commits since the last revision:

 - Review: Clarify RegisteredCharsets.java and add comments to test methods
 - Minor cleanup
 - Refactor IllegalCharsetName.java to use method source
 - Update EncDec.java to be more informative + cautious
 - Update data source: other -> standard Charsets
 - Review: Add comments to Contains.java to explain each test
 - Review: Add comment for test in AvailableCharsetNames.java
 - Review: Make CharsetContainmentTest.java data source and test method more clear

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14500/files
  - new: https://git.openjdk.org/jdk/pull/14500/files/eb374d64..6c7a8d05

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14500&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14500&range=01-02

  Stats: 108 lines in 6 files changed: 70 ins; 14 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/14500.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14500/head:pull/14500

PR: https://git.openjdk.org/jdk/pull/14500